The viruses provide a variety of structural types with which to explore the general mechanisms characterizing the assembly of macromolecular components and the regulation thereby of cellular growth and differentiation. This proposal outlines a relatively high- resolution X-ray diffraction analysis of two structures--bushy stunt virus (TBSV) and Sindbis virus. TBSV is the most suitable of the small viruses for X-ray crystallographic studies, and it is proposed to extend this work to a resolution of 8 A, using the method of isomorphous replacement already successful at 25 A. The resulting model will provide information about the distortions imposed on the protein subunits by quasi-equivalent bonding, about the packing of the RNA chain, and about protein-nucleic acid interactions. Sindbis is among the simplest and most regular of the lipid-containing viruses, a category that also includes the oncogenic RNA viruses. The lipid- containing viruses are models for understanding the structure and assembly of cell membranes and cell surfaces and the alteration of these structures that occurs in transformed cells. The proposal outlines approaches to the interpretation of the spherically-averaged X-ray diffraction from Sindbis that will indicate the nature of the packing of protein subunits in the surface of the particles, reveal further details of the lipid packing, and explore the question of in what way (if at all) protein bridges across the lipid bilayers.